This project was so unbelievably easy and free of all scary typical dye issues that I feel particularly satisfied that these onesies are being recycled and worn against the precious skin of our toddler. Stains around the neckline were promptly covered up and re-introduced the onesie back into the rotation.
During my late summer experiments, I used sassafras leaves, coreopsis blooms (each results in a yellow) and *old* daylily blossoms (which create a pale purple dye). The fabric 'fixer' was a vinegar:water bath (if you are doing food or berries - there is a different fixer). The dyeing process was just to simmer the plant goods in water for an hour and then add the fabric for however long you want - depending on the amount of saturation you need. You can fix the fabric and simmer the dye in two different pots at the same time - to keep things efficient. Which makes it an easy naptime project. A more comprehensive list of possible plants is here, along with specifics and how-tos.
The onesie pictured above was dyed with the sassafras and coreopsis combined - resulting in a muted soft yellow that is meant for baby wear. Autumn botanicals that can be used for dyes are juniper berries, pokeweed berries, acorns, sassafras leaves, wild grapes, sumac leaves (and fruit) and marigolds.